Reland of [ash-md] Fades overview header in and out

ash/common/wm/overview/scoped_overview_animation_settings.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_COMMON_WM_OVERVIEW_SCOPED_OVERVIEW_ANIMATION_SETTINGS_H_
 #define ASH_COMMON_WM_OVERVIEW_SCOPED_OVERVIEW_ANIMATION_SETTINGS_H_
 
+namespace ui {
+class ImplicitAnimationObserver;
+}  // namespace ui
+
 namespace ash {
 
 // ScopedOverviewAnimationSettings correctly configures the animation
 // settings for a WmWindow given an OverviewAnimationType.
 class ScopedOverviewAnimationSettings {
  public:
   virtual ~ScopedOverviewAnimationSettings() {}
+  virtual void AddObserver(ui::ImplicitAnimationObserver* observer) = 0;
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_WM_OVERVIEW_SCOPED_OVERVIEW_ANIMATION_SETTINGS_H_